Storm Water Management
The most effective, and possibly the only device for simply reducing or controlling storm water peak flow, is the storage basin—commonly known as a retention or detention basin. The term detention basin has come to be distinguished from a retention basin in that the latter is a storage device that has a normal pool of water such as a lake, pond or reservoir, while the detention basin is considered dedicated to its task and is normally empty. Both of these operate by the natural accumulation of storm water when a restriction, such as a weir or orifice, is placed on the flow.
These storage basins are typically used to mitigate storm water increases due to land development and are very effective when designed properly. For example, in a small watershed of 5 acres, for a shopping center that converts an existing wooded site to a land use consisting of pavement, the peak storm water flows can rise from 10 cfs to 20 cfs rather easily. In larger watersheds, proportional increases such as these could cause serious flooding and environmental damage.
The key criterion in storm water management is the limitation of after-development peak flows to rates equal to or less than the peak flows prior to development. In the example above, the developer of the shopping center would need to provide a storage basin to limit the after-development peak flows to 10 cfs. The developer may then need to provide substantial water quality treatment storage. Of course, the storage basin would occupy a significant portion of the site, typically ranging from five (5) to fifteen (15) percent or more of the development land area.
Many state and local municipalities normally require either control of storm water through written codes or insist on peak flow controls during the approval process. Whether or not storm water control is required, it is usually prudent to control storm water flows that are destined for off-site areas, merely to reduce the liability for damages in case of downstream flooding.
Storm Water Treatment
The treatment of storm water to improve water quality has gained considerable interest. Federal and state regulations now require storm water treatment for large sites and new Federal NPDES rules will require treatment from small sites. Further, some local municipal codes or environmental concerns mandate some form of storm water treatment for all sites.
A key criterion of storm water treatment is the capture of the first one-half (½) inch of runoff from newly disturbed areas within the watershed. The great majority of pollutants from runoff are contained in the first-flush. To treat the first-flush, the flows must be conveyed to specially designed water quality treatment basins where a variety of treatment processes take place, culminating with infiltration to the soil and/or evaporation. The water quality basins are designed particularly to capture only the first-flush of runoff, and to avoid the later segments of the runoff that would mix with and wash out the captured flow.
Our firm developed a simple design for a first-flush control device in 1990 that we have been using since on various engineering projects. Essentially, the control works on a hydraulic balancing principle—diverting the low flows to a water quality basin and then directing flows back to the drainage system when the water quality basin is full. The water quality basin is designed to store water for just a few days since an empty basin is necessary at the time of rainfall to fulfill the goal of water quality treatment.
Storm Water Storage Basin Theory
The method of computation used to design storm water storage systems is the straightforward and familiar application of conservation of mass principles—the volume flowing out is equal to the volume flowing into a system. This is known as the reservoir routing method, and a wide range of information is available on the subject in engineering and hydrology texts. A brief summation of the method is given here, as follows:
It is assumed for the numerical solution, that we are given the flow “Q” at every time interval “t”, being the series, Qin(t).Given: Vol(out)=Vol(in):
If a volume is allowed to accumulate (S), the modified mass equation accounts for this as follows:Vol(out)=Vol(in)−S
In a time interval t:Vol(out)/Δt = Vol(in)/Δt − ΔS/ΔtSince:Vol(out)/Δt = Qout(t)And, since:Vol(in)/Δt = Qin(t) andΔS = S(t)SubstitutingQout(t) = Qin(t) − ΔS/ΔtRearranging:S(t) = (Qin(t) − Qout(t)) × Δt(Eq. 1)
As described in words, the change in volume of storage within any time interval is equal to the rate of inflow in minus the rate of outflow, multiplied by the interval of time.
The outflow of a storage basin can be modeled by a non-linear hydraulic function, “g” relating head, or height (stage) “H” in the basin, and various physical characteristics of the control device; e.g., length of a weir or diameter of a pipe, referred to as the set “n”, and generally a constant “C”.
For example:Qout = C × g(n, H)(Eq. 2)
If the outflow of a storm water storage basin is restricted by a weir, the outflow function is as follows:Q=C×L×H^3/2 or Qout(t)=C×L×H(t)^3/2
Where:C is a factor (3.337)H is the flood stage in the basin inL is the weir length (ft)feet and H(t) is the height at any time
Further, there is a natural geometric relationship, or function “f” between height “H” and the volume “S” in the storage basin. This is often a tabular relationship between contour elevation and surface area that can readily be interpolated for storage volume at any height.
For example:H = f(S) or H(t) = f(S(t))(Eq. 3)
Equations 1, 2 and 3, above fully define the mathematics of the storage process that occurs in a detention or retention basin. The equations are easily solved by iterative techniques. The mathematical method is generally referred to by the generic term, reservoir routing, and it describes a relationship between inflow and outflow that can be seen graphically in FIG. 1.
It is important to note that the area between the inflow and outflow hydrograph is the exact equivalent of the storage volume reached in the storm water basin. Further, in the descending phase of the inflow, the area representing the outflow volume leaving the storage system is the 1 same as the inflow volume, unless some volume is captured within the system.
It is therefore an object of the invention to provide a system for reducing environmental impact of storm water flows, comprising a feed conduit, receiving storm water runoff; a bypass conduit; a detention basin; for reducing a net peak flow of storm water run off; a treatment basin, for removing pollutants from the storm water runoff; and a control system, receiving storm water runoff flow from the feed conduit, and splitting the flow between at least the detention basin, the treatment basin, and the bypass conduit, wherein a flow to the treatment basin is sensitive to a water level therein, a managed quantity of water flowing to the treatment basin until filled, and a remainder of the flow is split in a flow rate sensitive proportion to the bypass conduit and detention basin.
This system may operate in an environmental region, having a natural hydrograph, a development, situated within the environmental region, having a development hydrograph characterized by a higher and earlier peak flow than the natural hydrograph, wherein the system for reducing environmental impact of storm water flows delays the time of peak flow and reduces the level of peak flow of the development hydrograph, resulting in a mitigated hydrograph corresponding to the natural hydrograph.
It is also an object of the present invention to provide an environmental system, subject to storm water flows, comprising an environmental region, having a natural hydrograph, a development, situated within the environmental region, having a development hydrograph characterized by a higher and earlier peak flow than the natural hydrograph, a storm water runoff mitigation system, receiving storm water runoff from the development according to the development hydrograph, having a mitigated hydrograph, comprising:
(1) a bypass conduit,
(2) a detention basin, for reducing a net peak flow of storm water runoff;
(3) a treatment basin, for removing pollutants from the storm water runoff; and
(4) a control system, receiving storm water runoff flow, and splitting the flow between at least the detention basin, the treatment basin, and the bypass conduit, wherein a flow to the treatment basin is sensitive to a water level therein, a managed quantity of water flowing to the treatment basin until filled, and a remainder of the flow is split in a flow rate sensitive proportion to the bypass conduit and detention basin,
wherein the mitigated hydrograph has a peak flow rate at or below the natural hydrograph.
It is a further object of the invention to provide a method for reducing environmental impact of storm water flows, comprising receiving storm water runoff flow, and splitting the flow between at least a detention basin, a treatment basin, and a bypass conduit wherein a flow to the treatment basin is sensitive to a water level therein, a managed quantity of water flowing to the treatment basin until filled, and a remainder of the flow is split in a flow rate sensitive proportion to the bypass conduit and detention basin, the detention basin reducing a net peak flow of storm water runoff and the treatment basin removing pollutants from the storm water runoff.
The system may further comprise an outlet conduit, receiving flow from the detention basin and the bypass conduit. The control system may, for example, operate passively. A flow splitting may therefore occur passively. A partition of flows between the bypass conduit and the detention basin may be based on one or more of a respective pipe diameter, and a respective pipe height within a chamber. A partition of flows between the bypass conduit and the detention basin may be based on characteristics selected from the group consisting of one or more of a pipe diameter, a pipe height, orifice structure, and a weir structure. Under peak flow conditions, a water efflux rate from the bypass conduit and detention basin is preferably reduced by this system. A first flush runoff may be selectively shunted to the treatment basin. A treatment basin capacity may be established at level sufficient to hold a first flush volume plus an amount sufficient to minimize the aggregate volume of the detention basin and treatment basin, constrained by a predetermined peak flow efflux rate from an optimized combination of detention basin and bypass conduit characteristics; wherein the characteristic of the treatment basin, detention basin, and control system may be optimized through an iterative process. The control system is preferably optimized to reduce peak flows from the detention basin and bypass conduit according to the Army Corps of Engineers HEC-1 computer program. Preferably, the mitigated hydrograph models the natural hydrograph.